A Passive Optical Network (PON) is essentially an optical network that uses a single fiber optic cable for the transmission of signals from one point (e.g. a service provider) to a plurality of different points (e.g. customer premises). Most likely, the signals to be transmitted will be digital signals. Therefore, in addition to the fiber optic cable, the PON will necessarily include a component (i.e. modem) at the transmit end of the fiber optic cable that modulates digital signals onto a radio frequency (RF) carrier wave. The resulting RF signal is then converted into an optical signal for transmission over the fiber optic cable. At the receive end of the fiber optic cable, the process is reversed. Specifically, a component (modem) reconverts the optical signal to an RF signal, and then demodulates the RF signal for subsequent use.
An important aspect of a PON is that it can take advantage of the well known transmission of optical signals by Wavelength-Division Multiplexing (WDM). This essentially allows the PON to use one wavelength (λ1) for downstream traffic on the fiber optic cable, while simultaneously using another wavelength (λ2) for upstream traffic. Further, it is possible to have two or more upstream traffic wavelengths (e.g. λ1 and λ3), and two or more downstream traffic wavelengths (e.g. λ2 and λ4). This WDM capability, coupled with the point-to-multipoint characteristics of the PON, gives it a distinct advantage over other types of network architectures. Specifically, a PON configuration will reduce the amount of fiber optic cable that is required vis-à-vis a point to point architecture. A potential downside, however, is that fiber optic cables are known to introduce distortions into an optical signal that diminish its clarity.
Of all the distortions that may be introduced into an optical signal as it transits through a fiber optic cable, the most predominant distortion is the second order distortion. These second order distortions, however, are relatively easily identified. For example, consider an optical signal carrying RF frequencies fa and fb. It can happen that the fiber optic cable will induce two RF distortion signals at frequencies fa+fb and fa−fb into the optical signal as it transits through the fiber optic cable. In the case where fa≅fb, the second order distortions are fa+fb≅2fa and fa−fb≅0. In this case, fa−fb≅0 is trivial and 2fa defines the octave for fa.
In light of the above, an object of the present invention is to provide a passive optical network with a sub-octave filter that will transmit clear signals over the PON with minimal, if any, distortions at the receive end of the transmission. Another object of the present invention is to provide a passive optical network that effectively removes distortions from a transmitted signal that are induced into the signal by the fiber optic cable of the PON. Still another object of the present invention is to provide a passive optical network with a sub-octave filter for removing second order distortions from transmitted optical signals that is easy to use, is simple to employ and is comparatively cost effective.